http://www.youtube.com/watch?v=lwPe_84GBfY that was good and quite clear. I did it that way but actually did it in six sections. Now that I think of it, I probably could have done in with one long piece by mitering on the six edges that are straight. However, this worked okay.
When I had it all sewn, trimmed, turned and pressed, I topstitched about 1/4 inch from the edge. I figure that the trimming was pretty close and probably could come loose and fray. The topstitching should help.
(Click to enlarge)